Agglomerating moist particulate material by adjusting the zeta-potential to zero

ABSTRACT

A METHOD IN PELLETIZING PROCESSES FOR AGGLOMERATING MOIST PARTICULATE MATERIAL, THE METHOD COMPRISING THE STEPS OF NEUTRALIXING OR REMOVING THE ELECTRIC CHARGE OF INDIVIDUAL CONSTITUENT PARTICLES TO PREVENT THE REPULSION   OF SAID PARTICLES BY EACH OTHER, THEREBY TO PROVIDE A FIRMLY PACKED AGGLOMERATE.   D R A W I N G

April 3, 1973 P. G. KIHLSTEDT 3,7

AGGLOMERATING MOIST PARTICULATE MATERIAL BY ADJUSTING THE ZETAPOTENTIAL TO ZERO Filed May 19, 1970 Zp mV INVENTOR PER GUDMAR KIHLSTEDT BYa g 6 Z w ATTORNEYS United States Patent 3,725,033 AGGLOMERATING MOIST PARTICULATE MATE- gllgrlizlBY ADJUSTING THE ZETA-POTENTIAL TO Per Gudmar Kihlstedt, Bromma, Sweden, assignor to AB Cold Bound Pellets, Belestigen, Djursholm, Sweden Filed May 19, 1970, Ser. No. 38,856 Claims priority, application Sweden, May 27, 1969,

7,391/ Int. Cl. C21b 1/08; C22!) 1/28; B011 1/02 U.S. Cl. 75-3 7 Claims ABSTRACT OF THE DISCLOSURE A method in pelletizing processes for agglomerating moist particulate material, the method comprising the steps of neutralizing or removing the electric charge of individual constituent particles to prevent the repulsion of said particles by each other, thereby to provide a firmly packed agglomerate.

namely a first stage in which the particulate material is agglomerated by compressing the same into the form of briquettes, for example, or by tumbling the particulate material by means of rotating drums or plates in a man- -ner whereby nodules are formed, and a second stage in which the agglomerate is hardened for example, by creating hydraulic bonds at relatively low temperatures or by sintering in relatively high temperatures.

The present invention relates to the agglomeration of moist particulate material by pelletizing and more particularly to an advantageous method of procedure in the first mentioned step in pelletizing processes, by means of which non-hardened pellets are obtained which comprise very densely packed particles, thus imparting to the pellets in a simple and economic manner a high degree of mechanical strength which prevails both prior to and subsequent to the hardening process.

During the initial formation of nodules of moist particulate material, when pelletizing, the capillary forces between the particles and the liquid moistening said particles co-act to bring the particles together. This is clearly explained, among others, by Ilmoni and Tigerschitild in an article in Jernkontorets Annaler, 1950, pages 135-171. The liquid binding the particulate material together departs to a greater or lesser extent during subsequent treatment processes effected before and during the aforementioned hardening stage, during which processes the pellets are also subjected to high stresses and loads. Thus, as the moistening substance departs the agglomerate is held together to an ever decreasing extent by the capillary forces and the particulate material forming the pellets must therefore be boundtogether instead by atomic forces, i.e. the so-called Van der Waals forces, which, however, provide a fully satisfactory binding force only when the interstices of the particles are slight. In order to increase the capillary forces binding the particulate material together, and thereby facilitate the formation of pellets, it

held together even when the liquid moistening the particles has departed to a greater or lesser extent, and to ensure satisfactory mechanical strength of the pellets subsequent to hardening, it is proposed in accordance with the invention that the composition of the moist particulate material is adjusted so that the particles during the pelletizing process have substantially no electric charge. Exhaustive research, which resulted in the present invention, has shown that the individual particles in those minerals and other materials normally agglomerated are often negatively charged and repell each other, although other types of particles are used within the agglomerating technique which, at least under certain circumstances, are positively charged, and that for the purpose of obtaining a satisfactory bond between the particles, when pelletizing Without excessive grinding of the particles, it is necessary substantially to eliminate the repelling effect of the particles by removing or neutralizing their electric charge.

The electric charge of the moist particulate material or material mixtures normally used for pelletizing purposes is normally dependent on the pH of the moist material, and in the case of certain materials the particles within one or more pH ranges of the moist particulate material may be negatively charged and within one or more other pH ranges positively charged, and at a pH value or values at transition points between the aforementioned ranges may be completely without an electrical charge. When practising the invention on materials having these properties, the composition of the moist particulate materials is suitably adjusted by adjusting the pH of the liquid moistening the materials so that the particles are at least substantially de-charged.

De-charging of the particles can also be effected, in accordance with the invention, by using as a starting material mixtures of particulate material presenting different charging properties in moist condition, so that the electrical charges of the particles neutralize each other.

It is also within the scope of the invention to intermix with the particulate starting materials, for the purpose of de-charging the same small quantities of flocculating agents known from sedimentation techniques, for example flocculating agents of organic type, such as glue, and/ or conventional flocculating agents containing, for example, Al Fe or Ca +-ions.

It is also possible in accordance with the invention to combine the three aforementioned methods, that is adjustment of the pH value, intermixing of particle types which neutralize their respective electric charges and the use of flocculating agents. It is particularly suitable to combine pH adjustment with one or the other of the remaining methods, since the pH can easily be adjusted by adding a small quantity of a suitable acid or base substance. Whether or not the particles in the moist starting material have actually been decharged, can be determined by conventional methods such as by examining the flocculating properties of the starting material and/or by measuring the so-called z-potential and/or by measuring the pH. In this respect, determination of the pH of the material is often sufiicient, and it is then no longer necessary to determine the fiocculating properties or the z-potential of the material. The effect of the flocculating agents is normally dependent on the pH thereof, and similarly the z-potential often varies markedly with the pH value.

The accompanying drawings shows a diagram of the dependency of the z-potential on the pH value of a magnetite-hematite concentrate and a slag obtained from oxygen gas refinement of molten pig iron. The magnetitehematite concentrate was a mixture of 52.5 percent by weight magnetite and 47.5 percent by Weight hematite, the mixture containing 4.20 percent by weight SiO 0.82 percent by weight A1 2.66 percent by weight CaO, 1.08 percent by weight MgO, 63.4 percent by weight Fe, 0.08 percent by weight Mn, 0.67 percent by weight P, 0.014 percent by weight S and the remainder essentially chemically bound oxygen. The slag contained 6.8 percent by weight SiO 1.14 percent by weight A1 0 50.4 percent by weight CaO, 3.48 percent by weight MgO, 15.3 percent by weight Fe, 1.14 percent by weight Mn, 5.69 percent by weight P and the remainder essentially chemically bound oxygen. The curve 1 in the diagram illustrates the variation in z-potential of the magnetite-hematite mixture within the pH range of 8 to approximately 12, weak NaOH being used for varying the pH value. In a similar manner, the curve 2 in the diagram shows the variation in z-potential of the slag composition within the pH range 8 to approximately 12, weak H 80 being used for varying the pH of the slag.

As will be seen from curve 1 of the diagram, the magnetite-hematite concentrate can not be de-charged and made fully suitable for pelletizing simply by adjusting the pH. Since the magnetite-hematite concentrate and the slag, however, has substantially opposite charging properties they can be intermixed in such proportions and brought to such a pH value that the particles neutralize each other with respect to their electric charges. Curve 3 in the diagram shows the variation in z-potential with the pH value for a mixture comprising 90 percent by weight of the magnetite-hematite concentrate and 10 percent by weight of the slag, and also shows that this mixture of concentrate and slag is well suited for pelletizing purposes when its pH value has been adjusted to approximately 11.5. Weak H 50 was used for varying the pH of the mixture.

It will be understood that one skilled in the art can easily produce curves by laboratory experiments, similar to those shown in the drawing over the variation of the potential with change in pH for moist particulate material to be pelletized, and with the guidance of the curves select suitable pH adjusting or de-charging additives, whereby strong pellets can be obtained readily and economically without excessive grinding of the starting material. It will also be understood that not only the pH value but also other ion or flocculating agent concentrations can produce potential curves of the type shown on the drawing, and that such curves can also be produced within the scope of the invention and used as a guidance for controlling de-charging additives to the particulate material to be pelletized. When practising the invention the electric charge of the particles or the fiocculating properties or the pH value of the moist particulate material can be determined to advantage continuously or intermittently and the addition of solid and/or liquid decharging material can be controlled manually or automatically in dependence of the determined values.

Since only a basic binding agent can be envisaged with respect to the particulate concentrate chosen to illustrate the invention, the drawing only illustrates the potential variation for pH values above the neutral point, although it is advisable when an acid material or binding agent is used to examine also the potential variation for pH values below the neutral point.

What is claimed is:

1. In a process for agglomerating a quantity of moist particulate material selected from iron ore fines and mineral concentrates into a hard, crush-resistant pellet in which the moist particulate material together with an appropriate binder is first formed into a loosely agglomerated moist mass, and thereafter the loosely agglomerated mass is hardened into a crush-resistant pellet by either sintering at a relatively high temperature or by creating hydraulic bonds, the improvement for maximizing the bond strength of said pellets comprising: adjusting the zeta-potential of the material being agglomerated before it is formed into a loosely agglomerated moist mass so that it is substantially zero.

2. The process of claim 1 wherein the zeta-potential is adjusted either by:

(a) adding an acid or base to change the pH of the moist particulate material,

(b) adding particulate matter having an electrical charge of a different sign than said particulate material being agglomerated,

(c) adding a flocculating agent, or

(d) any combination of procedures (a), or (b) and 3. A process according to claim 1 wherein said moist particulate material being agglomerated is iron ore fines.

4. A process according to claim 1 wherein the moist particulate material being agglomerated is a particulate mineral concentrate.

5. The process according to claim 4 wherein said moist particulate material to be agglomerated is iron ore fines.

6. A process according to claim 4 wherein the moist particulate material to be agglomerated is particulate mineral concentrate.

7. In a process for agglomerating a quantity of a moist particulate material selected from iron ore fines and mineral concentrates into hard, crush-resistant pellets in which the moist particulate material together with an appropriate binder is first formed into loosely agglomerated moist masses and thereafter the loosely agglomerated masses are hardened into crush-resistant pellets by either sintering at a relatively high temperature or by creating hydraulic bonds, said process characterized in that before or during the hardening step a portion of the moisture holding the particulate material of each moist mass together departs from its mass, the improvement for maximizing the bond strength of said pellets comprising:

suspending a sample of the particulate matter to be agglomerated in water,

measuring the zeta-potential,

adjusting the zeta-potential of said suspended sample until it is substantially zero,

said adjusting accomplished by either:

(a) adding an acid or a base to change the pH of the moist particulate material,

(b) adding particulate matter having an electrical charge of a different sign than said material being agglomerated,

(c) adding a fiocculating agent to change the flocculating conditions of said particulate material,

(d) any combinations of procedures (a), (b), and

calculating the amount of acid, base, particulate matter of opposite charge, flocculating agent or mixtures thereof necessary to adjust the zeta-potential of the entire quantity of moist particulate material to be agglomerated to zero if said quantity were suspended in water, and

adding that amount of acid, base, particulate matter of opposite charge, fiocculating agent, or mixtures thereof to the quantity of moist particulate material to be agglomerated before it is formed into loosely agglomerated masses.

(References on following page) 5 References Cited UNITED STATES PATENTS Fishe 23239 Penney 23313 5 Federick et a1. 233 13 F-auser 25262.1

Drummond 23314 Lee 753 Crowe 753 Volin 753 6 FOREIGN PATENTS 1,195,468 6/1970 Great Britain 75-3 224,987 6/1958 Australia 753 OTHER REFERENCES Osipow: ACS Monograph 153, pages 81-96, 1961.

NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner 0 US. Cl. X.R. 23-3 13; 75-5 

